1. Field of the Invention
This invention relates to a method of and apparatus for removing gas from a vacuum enclosure by gettering and for introducing active getter into a vacuum enclosure such as may be employed for thermal insulation.
2. Description of the Prior Art
In the prior art methods of forming and maintaining vacuum in vacuum enclosures, it has been common practice to employ getter materials for sorption of gaseous components in the vacuum enclosure to achieve low gas pressures therein. The getter thus is disposed in the evacuable enclosure and has the primary function of taking up inleaking and evolving gases over the vacuum service life of the enclosure thereby continuously maintaining vacuum pressures at or below a predetermined low level. However, the getter may also be employed to reduce the initial evacuation pumping time necessary for attainment of the low vacuum operating low pressure in the system and to remove the gases and vapors initially present on the internal surfaces of the vacuum enclosure.
In systems where vacuum is employed to provide high quality thermal insulation, the use of getter materials is particularly advantageous. In these systems, the absolute pressure must be maintained below some specified upper limit; unless a high degree of evacuation is maintained, heat transmission in the insulation space by gaseous conduction may be excessive. Getter materials provide an effective means of achieving such levels of vacuum maintenance over prolonged periods of time, and accordingly, have been widely used in the thermal insulation art.
Despite their effectiveness, however, certain getter materials pose particular difficulty in handling due to their high activity, i.e., their tendency to strongly and spontaneously sorb gas under ambient conditions. With such active materials, any appreciable ambient exposure may result in rapid and significant decrease in the gas sorption capacity of the getter. The problem is especially severe where the getter is relatively expensive and supplied in small quantity, as is the case in many vacuum system applications. Moreover, many active getters exhibit relatively high heats of sorption, which may further aggravate the handling and installation problem.
Heretofore, the prior art has in some instances overcome the material handling problem associated with active getter materials by providing the getter in a relatively inert form. In this form the getter is disposed in the vacuum enclosure and thermally activated in situ after the vacuum enclosure has been evacuated and sealed. However, such methods generally require considerable direct or indirect heating involving extreme high temperature, which is impractical in many cases due to the likelihood of damage to the vacuum enclosure system.
The prior art has also provided active getter in fluid-tight packages which are installed in the vacuum enclosure and, after evacuation and sealing of the enclosure, are opened by indirect means. For example, in U.S. Pat. No. 3,108,706 to L. C. Matsch et. al and in U.S. Pat. No. 3,114,468 to A. W. Francis et. al, active getter is provided in a gas-tight vial under vacuum or inert atmosphere. The vial containing active getter is then installed in a thin-walled extension of the vacuum enclosure to be gettered. After evacuation and sealing of the vacuum enclosure, the walls of the extension are mechanically deformed until the vial is crushed, thereby exposing the getter and commencing gas sorption in the enclosure.
The above method, while generally providing efficient vacuum maintenance, has several associated disadvantages which limit its effectiveness. For example, in vacuum enclosures having metallic walls, breakage of the active getter-containing vial occasionally does not occur under deformation of the extension walls and visual vertification of the active-getter exposure is not possible. Extreme levels of deformation of the thin-walled extension may serve to increase the likelihood of active getter exposure, but may also rupture the thin-walled section and impair the leak-tightness of the vacuum enclosure. In addition, when vacuum enclosures processed according to the above method lose vacuum after extended period in service due to saturation of the getter, reconditioning or renewal of the vacuum system is necessary. In the renewal operation, the thin walled extension of the vacuum enclosure must be removed along with the crushed vial and saturated getter. The thin-walled extension must then be refabricated, secured to the vacuum enclosure, and tested for leak-tightness. Such procedure is time-consuming, tedious and expensive.
It is accordingly an object of the present invention to provide an improved method of and apparatus for removing gas from a vacuum enclosure by gettering.
Another object of the invention is to provide an improved method of and apparatus for exposing an active getter to a vacuum enclosure without in situ activation of the getter material.
A further object of the invention is to provide an improved container for active getter which may be opened for gas flow communication and exposure of the getter prior to sealing of the vacuum enclosure in which it is installed, without significant loss or impairment of sorptive capacity of the getter.
A still further object of the invention is to provide an improved method of and apparatus for using getter to re-establish and maintain vacuum in a vacuum enclosure which does not require refabrication of vacuum enclosure walls.
Other objects and advantages of the present invention will become apparent from the ensuing disclosure and appended claims.